Hydrogen clearance measurements of cerebral blood flow in shall animals have often been much lower than those reported using techniques which monitor radioactive iodoantipyrine or nicotine diffusion. We have shown that placement of a hydrogen electrode into gerbil striatum dramatically reduces blood flow throughout the impaled hemisphere, as measured by the alternative tracer diffusion methods. In chronic studies, blood flow recovered within several hours of electrode implantation. Blood flow in striatum determined using the implanted electrode increased in parallel with values obtained by radioactive tracer methods in the same animals, indicating that all methods provide accurate estimates of flow. Recent results indicate that a brief puncture of the cortical surface is sufficient to produce comparable disruption of hemispheric blood flow. Spreading depression shown to follow similar injury in other studies has been associated with reduced cerebrovascular reactivity to C02, and such a mechanism may in part account for the effect on blood flow observed in these studies.